Extruders for extruding thermoplastic and non-thermoplastic materials are available in different sizes and shapes and with one or more extrusion screws. This invention focuses on those extruders which have a single extrusion screw rotating in a tightly fitted barrel. This type of extruder utilizes the mechanical energy introduced through the screw shaft and the thermal energy from the heated barrel surface converting (plasticating) the thermoplastic material (solid polymer pellets or powder) into a uniform melt at a certain temperature and pressure. The agglomerated melted mass is then shaped into a body by being passed through a die assembly which extends the longitudinal axis adjacent to the barrel.
With an extruder of the extrusion screw type, continuous operation is possible, i.e., the process of loading the extruder with raw materials (pellets, for example) and extruding the finished product (extrudate) is relatively continuous. This is a desirable feature of this type of extruder. Therefore, in the continuous process, the extruder needs to convey, melt, mix and pump the material.
A disadvantage of the current state of the art with this type of extruder for the extrusion of mixed composite articles having a thermoplastic resin binder and non-thermoplastic powdered or granular material is that, as the mixed mass of material flows off the screw and into the die, and begins to cool and harden, the flights extending from the screw creates a spiraling "knit mark" along the outer surface of the material. This spiraling knit mark which travels along the axial length of the extrudate product results in a spiral weak area which has lower density and greater porosity. To demonstrate the inferior effect of the spiral "knit marks", an experiment was conducted on two extrudates (hollow cylinders). The first extrudate made using conventional parameters had spiral "knit marks", and the second made using this invention did not. The extrudate with the marks demonstrated a breaking strength of 198 lbs. The extrudate without the marks demonstrated a product approximately 40% stronger having a breaking strength of 276 lbs.
In the past, the primary solution to overcome the problem of spiral weak areas caused by the spiral "knit marks" has been to form the extrudate product with an overall greater density, i.e., the alternating spiral of greater density and lower porosity is made denser and less porous than necessary. However, this results in higher production costs due to increased material usage, lower production rates, increased extruder usage (wear and tear) and higher power consumption. It would therefore be desirable to have a way to eliminate these "knit marks" without the disadvantages noted above. A general object of this invention, then, is to provide a solution to the noted problem without the noted disadvantages.
A more specific object of this invention is to provide an extruder and an extrusion screw for the continuous extrusion of thermoplastic materials with non-thermoplastic materials to form a product without "knit marks" having a uniform density with consistent porosity.